The present invention relates to a microcomputer control system which is controlled on the basis of programs or data stored in memory. A control system in which an incorporated ROM can be modified in a pseudo manner is described in e.g., Japanese Published Unexamined Patent Application No. Hei 7-182153.
The control device comprises a ROM storing a control program for effecting a control procedure and a processing unit that executes the control program to control the device, and further includes a loadable and unloadable nonvolatile memory storing a replacement program to replace part of the control program and replacement specification information indicating whether to carry out the replacement, whereby the control program executes the replacement program in accordance with the replacement specification information. As a result, the program can be modified without replacing the original program ROM.
Although the above-described control device enables program modifications to be made without replacing an original program ROM, since the modifications are determined by replacement specification information stored in a nonvolatile memory, for example, when part of a control program in the original program ROM is modified to newly create an upgraded control unit and a nonvolatile memory is used therein, if the modified portion is replacement specification information, in the nonvolatile memory, for directing replacement with the replacement program, the modified portion would not be used. To prevent this requires an additional nonvolatile memory for the replacement program, provided for modifications to the control program of the program ROM.
As described above, the above-described control device has given no consideration to independent modifications between a control program in an incorporated RON and an external replacement program.